Photoelectric conversion elements are used in various photosensors, copying machines, solar cells, and the like. These photoelectric conversion elements have adopted various systems to be put into use, such as elements utilizing metals, elements utilizing semiconductors, elements utilizing organic pigments or dyes, or combinations of these elements. Among them, solar cells that make use of non-exhaustive solar energy do not necessitate fuels, and full-fledged practicalization of solar cells as an inexhaustible clean energy is being highly expected. Among these, research and development of silicon-based solar cells have long been in progress. Many countries also support policy-wise considerations, and thus dissemination of silicon-based solar cells is still in progress. However, silicon is an inorganic material, and has limitations per se in terms of throughput and molecular modification.
Under such circumstances, research is being vigorously carried out on dye-sensitized solar cells. Particularly, Graetzel et al. at l'Ecole Polytechnique de l'Universite de Lausanne in Switzerland have developed a dye-sensitized solar cell in which a dye formed from a ruthenium complex is fixed at the surface of a porous titanium oxide thin film, and have realized a conversion efficiency that is comparable to that of amorphous silicon. Thus, the dye-sensitized solar cells instantly attracted the attention of researchers all over the world.
Dye-sensitized photoelectric conversion elements making use of semiconductor fine particles sensitized by a ruthenium complex dye, to which the foregoing technology has been applied, are described (see, for example, Patent Literature 1). Generally, these photoelectric conversion elements using the semiconductor fine particles sensitized by a dye, such as a ruthenium complex dye, are produced by applying a high-viscosity dispersion liquid containing semiconductor fine particles on an electrode support, volatilizing the solvent contained in the applied dispersion liquid at a high temperature (e.g., 400° C. to 500° C.), and adsorbing a dye to the semiconductor fine particles. However, the time or energy consumed in this solvent volatilization process poses an obstruction to cost reduction in production of a photoelectric conversion element and a photoelectrochemical cell.
In regard to this problem, Patent Literature 2 describes a method of adsorbing a dye to semiconductor fine particles, by applying on a support a dispersion liquid in which the content of additives excluding semiconductor fine particles and dispersion solvent is 1% by mass or less of the dispersion liquid, and heating the dispersion liquid coating at 250° C. or below.
The photoelectric conversion element is required to have high initial conversion efficiency and both less reduction in conversion efficiency and excellent durability even after use. In terms of durability, however, it is not enough to produce the photoelectric conversion element using semiconductor fine particles simply sensitized with an ordinary dye in accordance with a method described in Patent Literature 2.